1. One Field of the Invention
The present invention is in the field of knee ligament testing systems, and it relates particularly to apparatus and methods for measuring anterior and posterior drawer shift or displacement of the proximal tibia relative to the distal femur as a result of applied forces so as to determine the presence or absence of a torn anterior or posterior cruciate ligament, respectively.
2. Description of the Prior Art
The clinical test which is the most similar to the method of the present invention for determining a disrupted or torn anterior cruciate ligament is known as the "Lachman" test. The Lachman test has been taught for many years by John W. Lachman, Chairman and Professor of Orthopedic Surgery at Temple University. The Lachman test is described in considerable detail by Torg, J. S., Conrad, W., and Kalen, V. in an article entitled "Clinical Diagnosis of Anterior Cruciate Ligament Instability in the Athlete", in The American Journal of Sports Medicine, Vol. 4, No. 2, March/April, 1976 at pages 84-93. In performing the Lachman test, the patient lies supine on a table with the knee held between full extension and 15.degree. of flexion. The person performing the test holds the thigh in one hand and the calf in the other, applying a firm anterior pressure to the calf in an attempt to translate the proximal tibia anteriorly relative to the distal femur. A positive test which indicates a torn or disrupted anterior cruciate ligament is one in which there is proprioceptive and/or visual anterior translation of the tibia in relation to the femur with a characteristic "mushy" or "soft" end point. This is in contrast to a definite "hard" end point elicited when the anterior cruciate ligament is intact.
The principal failing of the Lachman test is that it is entirely subjective and provides no measurement of knee displacement. Thus, the Lachman test provides no objective basis for comparison either with the sound knee of the patient or with statistical information relating the amount of displacement to the condition of the anterior cruciate ligament. The entirely subjective nature of the Lachman test also precludes its use in the development of any statistical information comparing anterior drawer shifts between uninjured and injured knees.
Another problem with the Lachman test is that the required grasping of the thigh and calf in the hands of the tester tends to cause patient guarding or resistance by muscular contraction, which will mask the true condition of the anterior cruciate ligament. The Lachman test is useless in the case of an acute knee injury (within seven days of the occurrence of the injury) when the knee joint is usually still swollen and painful, which greatly increases patient guarding. Further problems with the Lachman test are that a person with small hands has difficulty performing this test; and even with large hands the thigh and calf of an obese person cannot be adequately grasped to perform the test.
The first system capable of measuring or documenting the anterior or posterior displacement of the proximal tibia relative to the distal femur was that of J. C. Kennedy and P. J. Fowler, who described their system in an article entitled "Medial and Anterior Instability of the Knee. An Anatomical and Clinical Study Using Stress Machines," Journal of Bone and Joint Surgery, Vol. 53-A, 1971, at pages 1257-1270. According to the Kennedy ahd Fowler system, the patient was strapped upright into an immovable seat resembling a hydraulic barber's chair and the foot strapped down with the knee bent to 90.degree. of flexion. An anterior pulling force or a posterior pushing force was applied to the proximal tibia by gas-pressurized actuator cylinders. X-Rays were taken of the tibia and femur relative positions before and after the force was applied, and the displacements were recorded by directly measuring the motions seen on the X-Ray films. This system had the disadvantage that it had no direct mechanical means for indicating or measuring the tibial displacements relative to the femur as they occurred, as well as the inherent disadvantages of X-Rays. Another, major disadvantage of this system was that it attempted to immobilize the femur in what can be considered to be an earth-based or chair-based system. The problem with such reliance upon the supposed immobilization of the femur is that the femur is encased in a large amount of muscle tissue which, when relaxed as required for such testing, provides no firm locational support for the femur. On the other hand, if the muscles are tight enough for a somewhat rigid supporting of the femur, then the muscle tightness opposes the tibia/femur displacement which is the objective of the test.
A still further failing of the Kennedy and Fowler system was that their equipment was only capable of measuring knee displacements at 90.degree. of flexion. Applicant has found that while 90.degree. of flexion is sometimes satisfactory for posterior drawer testing, it is quite unsatisfactory for anterior drawer testing because of the large amount of posterior leverage which the powerful hamstring muscles have in opposition to the anterior drawer test, whereby only a slight amount of hamstring guarding will mask the anterior drawer test at large angles of knee flexion such as 90.degree.. This problem is not present in the Lachman anterior drawer test, wherein the knee is between full extension and 15.degree. of flexion.
In any earth-based or chair-based system such as that of Kennedy and Fowler, the imposing nature of the large and complex equipment, the discomfort of the apparatus employed to strap the patient down, and then the large amount of time, usually at least about 11/2 hours, required to strap the patient down, adjust the equipment and take the tests, inevitably caused patient guarding that interfered with the accuracy of the tests.
Markolf, K. L., Graff-Radford, A., and Amstutz, H. C., reported the fitst equipment that was capable of indicating or recording anterior/posterior tibial force versus displacement. This was reported in an article entitled "In Vivo Knee Stability", in the Journal of Bone and Joint Surgery, Vol. 60-A, No. 5, July, 1978, at pages 664-674. In this system a handle attached to a force transducer is strapped around the patient's calf. The examiner manually pulls or pushes through the handle which senses the force that is being applied as a displacement transducer records the displacement of the proximal tibia relative to the patella on the femur.
The Markolf et al system is again an earth- or chair-based system which has the same failings as those pointed out hereinabove for the Kennedy and Fowler system. In the Markolf et al system the reference point for all tibial displacement measurements is the chair, so that considerable equipment is directed toward an attempt to immobilize the patient's femur. Thus, there is a system of inflatable thigh compression pads, a contoured patellar compression block, and compression of the patient's sacrum by the rigid back of the chair. Additionally, the patient's foot is strapped down. As with the Kennedy and Fowler apparatus, the Markolf et al apparatus, due to its large, complex and imposing nature, the discomfort of the strapped-down patient, and the approximately 11/2 hours required to set up and perform the testing, caused considerable patient guarding that seriously interfered with the testing.
Applicant is aware of only one other type of apparatus which seeks to determine the presence or absence of a knee ligament deficit condition. This was reported by Crowninshield, R.; Pope, M. H., Johnson, R,; and Miller, R. in an article entitled "The Impedance of the Human Knee" in the Journal of Biomechanics, Vol. 9, 1976 at pages 529-535. This was laboratory apparatus which applied a cyclic rotational motion about the longitudinal axis of the tibia. By varying the frequency of the mechanical oscillation of the tibia while at the same time attempting to keep the femur relatively stable, the mechanical impedance characteristics of the knee were measured. These included the resonant frequency of oscillation of the knee, the change in the phase lag between the force input cycle and the displacement output cycle with varying frequency, and the like. Since the Crowninshield et al apparatus imposed only rotational or varus-valgus (medial and lateral bending) motions on the knee, this equipment was not capable of indicating or measuring anterior or posterior tibia/femur drawer shift to determine whether or not the anterior and posterior cruciate ligaments were torn.
A general failing in the art was the inability of any of the prior art systems to test the condition of anterior or posterior cruciate ligaments in the case of an acute knee injury, i.e., within seven days of the occurrence. This was primarily because of guarding that resulted either from hand manipulation that was required for the testing or from reaction to the discomfort of equipment used to strap down the patient's leg.